The present invention relates to the fabrication of thin crystalline films on temperature sensitive substrates by laser irradiation. More specifically, but without limitation thereto, the present invention relates to the fabrication of a thin monocrystalline silicon film on a substrate using a pulsed excimer laser and a unique arrangement of optics to control the temperature budget.
Current flat panel display technology uses liquid crystal displays integrated with thin film transistors (TFT's). Almost all active matrix LCD's comprise amorphous silicon on glass. For the next generation of active matrix LCD's, polycrystalline silicon is being considered instead of amorphous silicon. The performance of polycrystalline silicon TFT's could be further improved by improving the crystalline structure of silicon films. Fabrication of device quality semiconductor films on amorphous substrates such as glass is also important for low cost, disposable electronics applications including non-planar or curved surfaces and flexible substrates. CW lasers have been used with a contoured beam for improving the crystallinity of silicon films on amorphous surfaces, but this approach does not accommodate current needs for large area, rapid processing, and low thermal budget processing.
U.S. Pat. No. 4,707,217 issued on Nov. 17, 1987 to Aklufi teaches that single crystal growth on amorphous or polycrystalline substrates is thermodynamically favored under specific conditions, and that single crystal growth may be obtained by maintaining the angle defined by the trailing liquid edge of the scanned hot zone smaller than the included angle defined by the intersection of the preferred growth planes of the crystal (col. 1, ln 57-65). Aklufi further teaches establishing a layer of a semiconductor material such as silicon or any other material that has a crystalline state, and then continuously scanning the surface of the material with a wedge-shaped heat-zone created by a heat source such that the angle of the heat source is less than the angle defined by the intersection of the preferred growth planes. A portion of the material is melted along the apex of the scan and solidifies along the trailing edge of the scan into single crystal material.
A continuing need exists for a low thermal budget process to improve crystallinity of silicon on amorphous surfaces that may be non-planar and flexible, and to provide lateral crystal growth (DLC) in thin films.